Gas turbines offer lower weight than a piston engine of equal power output, but at high rotational speed and cost—needing gear-reduction for direct application in place of a piston engine. Despite low mechanical friction, existing turbines have internal aerodynamic drag of the multiple stages of rotor blades revolving at high velocity. Mechanical balance is critical.
Geared gas turbines power helicopters, turboprop business planes and turboprop short-field transports; turbojets power fighters, and turbofans power airliners. Piercing whine, an original drawback of airborne turbines, has over the years been transformed into a satisfactory swish.
The piston engine offers moderate cost and high utility, but its mechanical friction seriously limits efficiency. A study by engine-pioneer Earle Ryder of heat rejection in a cylinder of a Pratt & Whitney 28-cylinder, 3,500 hp radial aircraft-engine reported, “The amount of heat that must be transferred to the cooling air is equal to about two-thirds of the energy delivered to the propeller during lean, low-power cruse.”*
Accordingly,                Energy output to the propeller=Ep         Heat wasted=⅔Ep         Therefore, Ep+⅔Ep=Efuel                      5/3Ep=Efuel             ∴ Ep=⅗Efuel * “Technicalities” by Peter Garrison, FLYING Magazine, November 1993 issue, Page 99.                        
That highly developed heavy-aircraft piston engine's energy-efficiency was only 60%: it wasted 40 percent of its fuel.
The low efficiency of piston engines originates in viscous friction of the lubricated pistons and piston rings sliding inside the cylinders.
Cooling methods to dispose of the frictional heat further subtract from the fuel energy: the stout metal structure that supports the fuel-heated combustion chambers conducts heat toward necessarily cooled moving parts lubricated by motor oil which decomposes at elevated temperatures.
In piston engines of many cylinders, the power-stroke of one cylinder helpfully overlaps the compression-stroke of another cylinder. In light-aircraft piston engines of few cylinders, propeller-inertia sustains rotation at idling speed. Automotive piston engines need a compact therefore massive flywheel.
Aviation piston engines serve at a wide range of torque but a moderate range of shaft rotational speed; consequently their dual ignition systems and other accessories are simple and reliable.
Automotive gasoline-fueled piston engines' ignition timing and fuel-air mixture are less simple on account of the widely variable loads and driving speeds. These design factors have been refined satisfactorily; mechanical and electrical failures infrequently occur among the somewhat complicated accessories. Wholly efficient combustion of fuel would exhaust pure carbon dioxide, yet contemporary gasoline-fueled piston engines exhaust some poisonous carbon monoxide and a significant unburned portion of the fuel. This necessitates pollution-reduction accessories.
Diesel piston engines are comparatively long-lived and efficient in service, burning the injected fuel centrally in the combustion chambers' hotly compressed air and thus quite thoroughly, causing minimal pollution and avoiding fuel-dilution of the lubricant on cylinder-walls. The necessary high compression ratio requires robust weighty mechanical parts. Diesels need accurately timed high-pressure fuel injection, also glow-plugs to enable cold startups; to cool the fuel injectors, fuel has to circulate continuously through them.
Plainly, piston engines arrived at usefulness through unbounded human tenacity.
Long confronted was the puzzle of how to invent a turbine engine having the positive compression-displacement of piston engines while avoiding their immoderate mechanical friction-losses.
Without intake and exhaust valves, without internal aerodynamic drag of multitudes of turbine blades fanning the air, without gear-reduction it would serve at the same convenient range of rotational speeds as piston engines.